Electronic Siren Circuit

ABSTRACT

The invention is an electronic siren comprising in combination means for generating periodically occurring charging pulses at a selected one of a plurality of predetermined repetition rates, time-constant determining means coupled to the pulse generating means for generating a repeating voltage wave form in response to the charging pulses and means coupled to the time-constant determining means for generating a square wave signal having a frequency proportional to the instantaneous voltage of the voltage wave form. Coupled to the square wave generator is an output circuit which includes a first pair of transistors and first circuit means coupling the transistors to the square wave generator for driving the first pair of transistors individually and alternately into saturation in response to the aforementioned square wave signal. There are also provided a second pair of transistors and second circuit means coupling the second pair of transistors to the first pair of transistors for driving the second pair of transistors individually and alternately into saturation in synchronism with saturation of the first pair of transistors.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electronic siren circuits and moreparticularly is an improved electronic siren circuit incorporating asquare wave generator and an output circuit which utilizes diffusedbased transistors having a fast "rise" time and which are driven sharplybetween non-conducting and saturated states whereby heat and powerlosses within the circuit are minimized.

2. Description of the Prior Art

Prior art electronic siren circuits, such as that disclosed in U.S. Pat.No. 3,051,944 to the present Inventor, typically include a voltagecontrolled variable frequency oscillator which generates a square waveoutput signal, the square wave having a repetition rate or frequency inthe audio frequency range. These circuits further include voltage signalgenerating circuits which apply a selected one of a plurality ofdifferent wave forms to the variable frequency oscillator to produce thedesired siren signal. The output of the voltage controlled variablefrequency oscillator is then applied to a speaker through a suitablepower amplifier.

In the majority of applications, the electronic siren is installed in amotor vehicle or similar mobile unit. Consequently, the power and spaceavailable to operate and install the siren is limited. For this reason,and due to the need to produce a siren sound having a sufficient volumeto be heard above normal ambient sound levels, it is important that thesiren circuit produce the desired siren sounds with maximum efficiency.In this regard, prior art electronic siren circuits exhibit some loss ofefficiency which results from rounding off of the square wavesound-generating signals produced therein.

SUMMARY OF THE INVENTION

The invention in its broader aspects is an electronic siren circuitwhich comprises means for generating a selected one of a plurality ofpulse trains having different predetermined repetition rates, timeconstant determining means coupled to the charging pulse generatingmeans for generating a predetermined repeating voltage signal inresponse to the charging pulses, and a voltage controlled variablefrequency oscillator coupled to the time constant determining means forgenerating a variable frequency square wave signal having a frequency inthe audio frequency range and adapted for producing the desired sirensound. Coupled to the square wave generator is a power output circuitwhich includes a first pair of .[.diffused based.]. transistors coupledin a push-pull configuration and first circuit means coupling the firstpair of transistors to the square wave generating means for driving thefirst pair of transistors alternately and individually into saturationin response to the square wave signal and a second pair of diffusedbased transistors and second circuit means coupling the second pair oftransistors to the first pair of transistors for individually andalternately driving the second pair of transistors into saturation insynchronism with saturation of the respective ones of the first pair oftransistors. The output circuit further includes impedance matchingmeans for coupling a loudspeaker or other sound reproducing devicesthereto.

In a specific embodiment, there are also provided means for selectivelyconnecting an audio signal from a radio or microphone to the first pairof transistors and biasing the first pair of transistors to theirthreshold of conductivity, whereby the first pair of transistors, andthe second pair of transistors which operate in synchronism therewithare rendered individually and alternately proportionately conductive inresponse to the audio signal.

In another specific embodiment the electornic siren circuit of thepresent invention further includes a first isolation amplifier coupledbetween the time and constant determining means and the square wavegenerator and a second isolation amplifier coupled between the squarewave generator and the power output circuit to reduce undesirable signalfeedback and to facilitate use of the circuit for driving an auxiliarysiren device.

In yet another specific embodiment, the square wave generating meansincludes a pulse-shaping network for further reducing "rounding off" ofthe generated square wave signal and a balance control means forrendering the square wave signal symmetrical.

It is therefore an object of the invention to provide an improvedelectronic siren circuit having increased power efficiency;

It is another object of the invention to provide an electronic sirencircuit which exhibits less "rounding off" of the generated square wavesignal;

It is still another object of the invention to provide an electronicsiren circuit which utilizes diffused based transistors operatedalternately between nonconductive and fully saturated states;

It is yet another object of the invention to provide such a circuitwhich includes means for rendering the square wave signal symmetrical;

It is still another object of the invention to provide an electronicsiren circuit having an output circuit which can selectively operateboth as a high efficiency square wave amplifier and as a high power lowdistortion audio amplifier;

It is another object of the invention to provide such a circuit whichincludes means for reducing signal feedback and enable the use of thecircuit with auxiliary equipment.

DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent and the inventionitself will be best understood by reference to the following descriptionof an embodiment of the invention taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a block diagram of an electronic siren circuit in accordancewith the present invention;

FIG. 2 is an electrical schematic diagram of the electronic sirencircuit of the present invention; and

FIG. 3 is a diagram showing wave forms occurring in the electronic sirenof the invention and useful in explaining the operation thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, there is illustrated in FIG. 1 a blockdiagram of an electronic siren 10 comprising a pulse train generator 12,a time constant circuit 14 electrically coupled thereto, a voltagecontrolled variable frequency oscillator 16 (hereinafter referred to asVFO 16) which is coupled to the time constant circuit 14 and a poweramplifier 18 which is driven by VFO 16. Connected to the power amplifier18 is a conventional speaker 20. As will be explained in detail below,pulse train generator 12 generates a selected one of a plurality ofpulse trains having different predetermined repetition rates. The pulsesfrom pulse train generator 12 are applied to the time constant circuit14 which generates a repeating voltage wave form. The voltage wave formis in turn applied to the VFO 16 which generates a square wave signalhaving a frequency proportional to the voltage amplitude appliedthereto. The output signal from the VFO 16 is then applied to a poweramplifier 18 wherein the signal is amplified and applied to the speaker20 to produce the desired siren tones.

Referring now to FIG. 2, pulse train generator 12 is a multivibratoroscillator circuit which includes a first oscillator transistor 30having its emitter 31 coupled to a source of regulated direct currentvoltage 32 through a resistor 34 and a positive voltage supply buss 36and its collector 33 connected to ground through resistor 39. Anoperating bias voltage is applied to the base 35 of oscillatortransistor 30 by means of a voltage divider comprising resistors 38 and40. A second oscillator transistor 42 has its emitter 43 connected topositive buss 36 through a resistor 44 its collector 45 connected toground, and its base 46 coupled to ground via a biasing resistor 39,respectively.

A first six position switch 48 has its armature 50 connected directly tothe emitter 31 of transistor 30. Switch 48 further includes poleterminals 1a, 2a, and 3a which are connected to positive buss 36, andpole terminals 4a, 5a, and 6a which are connected to the emitter 43 ofoscillator transistor 42 through capacitors 52, 54, and 56 respectively.

The output terminal 60 of pulse train generator 12 is connected to timeconstant circuit 14 which comprises a diode 68 having its cathode 69connected to output terminal 60 and a resistor 70 connected electricallyin series with the anode 71 of diode 68 and output terminal 92. A secondsix position switch 72 is ganged with switch 48 and includes an armature74 which is connected to output terminal 92. Switch 72 further includespole terminals 1b and 2b which are connected to positive buss 36,terminals 3b and 4b, connected to positive buss 36 through a capacitor76, terminal 5b, connected to positive buss 36 through capacitor 78, andterminal 6b which is connected to the center of a voltage dividernetwork comprising resistors 80 and 82, the latter having their oppositeends connected to the positive buss 36 and ground, respectively. Acharging resistor 90 is connected between output terminal 92 andpositive buss 36 as shown. Also connected to terminal 92 is a resistor84, a normally open switch 86, and an auxiliary siren terminal 88, asshown, for a purpose to be explained below.

The output signal from time constant circuit 14 appears at terminal 92and is applied to the base 99 of an isolation amplifier transistor 100.Transistor 100 has its emitter 101 connected to positive buss 36 viaload resistor 102 and its collector 103 connected to ground andfunctions to both amplify the signal appearing at the output terminal 92and to provide impedance isolation between time constant circuit 14 andVFO 16.

VFO 16 is an astable multivibrator circuit which includes a pair ofswitching transistors 110, 112 which have their emitters 114, 116connected in common and to supply buss 36 via resistor 118. Thecollector 120 of switching transistor 112 is coupled to ground viaserially connected resistor 122 and variable resistor 124. The collector125 of switching transistor 110 is coupled to ground through seriallyconnected load resistors 126 and 128. Connected between the base 130 ofswitching transistor 110 and collector 120 of switching transistor 112is a frequency determining capacitor 132 and a similar frequencydetermining capacitor 133 is connected between the base 134 of switchingtransistor 112 and the collector 125 of switching transistor 110.Resistors 140, 142 are connected electrically in series between base 130of switching transistor 110 and the emitter 101 of isolation transistor100 and a clamping diode 144 is connected between the common connectionof resistors 140, 142 and the collector 120 of switching transistor 112as shown. Resistors 146 and 148 and a diode 150 are similarly connectedbetween the base 134 of switching transistor 112, the emitter 101 ofisolation transistor 100, and the collector 125 of switching transistor110.

Connected in parallel with resistor 126 is a pulse shaping capacitor160.

The output signal from the VFO 16 appearing at output terminal 162thereof will be a square wave signal having a frequency which isdirectly proportional to the voltage applied to its input terminal 164and of essentially constant power.

Connected to output terminal 162 of VFO 16 is a second isolationamplifier 166 which includes a double transistor connected in a"Darlington" configuration. The amplifier 166 has an input terminal 168connected to output terminal 162 of VFO 16, an output terminal 172, anda grounded terminal 170.

Power amplifier circuit 18 comprises a first pair of .[.diffused base.].power transistors 180 and 182 which are connected together in apush-pull configuration wherein their emitters 184, 186 are connected incommon and to ground via a resistor 188. Collector 190 of powertransistor 180 is connected to one terminal 192 of the primary winding193 of an auto transformer 194 and collector 196 of power transistor 182is correspondingly connected to the other primary terminal 198 of autotransformer 194. The center tap 200 is connected to the center of avoltage divider circuit which includes resistors 201, 203 which are inturn connected directly to the B+ supply 32 and ground, respectively.Bases 202, 204 of power transistors 180, 182, respectively, areconnected to opposite ends of the output winding 206 of a couplingtransformer 208. A temperature compensated network which includesparallel connected resistor 212 and a temperature variable resistor(thermistor) 214, which has a negative temperature coefficient form avoltage divider circuit connected between B+ supply 32 and ground whichapplies a predetermined bias voltage to the bases of transistors.[.100.]. .Iadd.180.Iaddend., 182 via the center tap 207 of couplingtransformer 208. The values of resistors 188, 210, 212, 201 and 203 andthermistor 214 are selected such that transistors 180, 182 arestatically biased to their threshold of conductivity, that is, to apoint where transistors 180, 182 are just turned "off."

The oppositely disposed terminals of the input winding 216 oftransformers 208 are connected to respective ones of the armatures 220and 222 of third and fourth six position switches 224, 226,respectively. which are also ganged with switches 48 and 72. Aconventional volume control network 230 is connected across terminals 1cand 1d of switches 224, 226, respectively. Volume control network 230includes input terminals 232 which provide a means for connecting thepower amplifier 18 to radio receiver output terminals (not shown) inconventional manner.

Similarly, a microphone 234 is connected across terminals 2c and 2d,there being a conventional load resistor 236 and volume control resistor238 connected therebetween as shown. Microphone 234 includes a momentarytype "on-off" switch 240 connected in series therewith. Switch 240 isalso provided with a second pair of normally open, momentary contacts242 and a jumper 244 may be connected electrically in series therewithbetween the supply buss 36 and the armature 74 of switch 72 in the timeconstant circuit 14 and a jumper 245 connected between switch 226terminal 2c and switch 226 terminal 2d for reasons to be explainedbelow.

The output terminal 172 of second isolation amplifier 166 is connectedto terminals 3c through 6c of switch 224 via a suitable load resistor248, there being a capacitor 250 connected between terminals 3c through6c and the supply buss 36 which removes or clips any spikes from thesquare wave signal generated in VFO 16 and prevents ringing. Terminals2d through 6d switch 226 are connected directly to the supply buss 36.

The output terminals 260, 262 of auto transformer 194 are connectedrespectively to the bases 264, 266 of a second pair of diffused basepower transistors 268, 270, respectively. Both power transistors 268 and270 have their collectors 271, 273, respectively, connected to groundand their emitters 275, 277, connected to opposite ones of the inputterminals 279, 281, of impedance matching transformer 272. Center tap283 of output transformer 272 is connected to the B+ supply line 32 at274 and the output terminals 276, 278 are connected to output terminals280 of the siren circuit 10. It will again be observed that theinterconnection of the second pair of power transistors 268, 270 enablesthem to operate in push-pull mode in synchronism with the operation ofpower transistors 180, 182.

The B+ supply 32 for the siren circuit 10 includes an input terminal 282connected to any suitable source of positive potential such as a vehiclebattery, a fuse 284 and "on-off" switch 286 connected in seriestherewith. An indicator light 288 is connected between "on-off" switch286 and ground to indicate when the siren circuit has been turned "on."Also connected to "on-off" switch 286 are a pair of filter capacitors290, 292 and a zener diode 293 having its anode connected to ground andits cathode connected to the positive buss 36. A ballast light 300 isconnected electrically in series between the cathode of zener diode 293and "on-off" switch 286 to provide better voltage regulation andoverload protection of zener diode .[.243.]. .Iadd.293.Iaddend..

The operation of the electronic siren circuit 10 is as follows. Pulsetrain generator 12, as stated, is a multivibrator oscillator circuit.When armature 50 of switch 48 is in position 4a, 5a, or 6a, capacitors52, 54, and 56, respectively, will be connected between the emitter ofoscillator transistor 42 and the emitter of oscillator transistor 30. Inthis mode, the oscillator will generate a plurality of trains ofrecurring pulses appearing at output terminal 60 and shown,respectively, as wave forms A-1, A-2, and A-3 of FIG. 3. When switch 48has its armature 50 in positions 1a, 2a, or 3a, the B+ supply voltagewill be applied directly to the emitter of transistor 30 thereby biasingtransistor 30 to an "on" condition. Because of the direct connection,transistor 30 will remain in this conductive state and pulse traingenerator 12 will produce a constant DC voltage at its output terminal60.

When switch 72 has its armature 74 in positions 1b or 2b, DC supply 32potential will be applied directly to the base 99 of isolationtransistor 100 rendering this transistor non-conductive. When armature74 of switch 72 is in positions 3b, or 4b, capacitor 76 will beconnected to the B+ supply via resistor 90 and to the output terminal 60of the pulse train generator 12 via resistor 70 and diode 68.Correspondingly, whenever the output from the pulse train generator 12is at a high level, such as for example between time t₀ and t₁ in waveform A-1, or whenever oscillator transistor 42 is non-conductive as aresult of transistor 30 being locked in a conducting state, a reversebias voltage will be applied to the cathode 69 of diode 68. Reversebiasing diode 68 in turn causes a charging current to pass throughresistor 90 and into capacitor 76 thereby equalizing the voltage at theopposite terminals thereof and effectively removing any charge thereon.Conversely, whenever the wave form from pulse train generator circuit 12is at a low level, such as for example between times t₁ and t₂ of waveform A-1, the capacitor 76 will begin to devolop a charge across itsterminals. This charging action will correspondingly produce a negativeexponential wave form having a predetermined time constant at outputterminal 92. Since, as described above, pulse train generator 12 isrendered inactive and oscillator transistor 42 statically nonconductivewhen switch armature 50 is in the 1a, 2a, and 3a positions, it will beseen that when switch 72 also has its armature 74 in the 1b, 2b, and 3bpositions, the capacitor 76 will eventually become fully charged and theoutput signal appearing at output terminal 92 will be direct currentvoltage.

It will now be observed that when switch armature 74 is in the 3bposition, manual closing of switch 86 will cause capacitor 76 to charge.Conversely, when switch 86 is opened, capacitor 76 will dischargethrough resistor 90. Thus, when switch armature 74 is in the 3bposition, switch 86 provides a means for generating decreasing andincreasing exponential voltage wave forms at terminal 92 each time it isclosed and opened, respectively, with the time constant being determinedby capacitor 76 and resistor 84.

When switch 72 has its armature 74 in the 4b and 5b positions and switch48 is in the 4a and 5a positions, respectively, capacitors 76 and 78will be connected to the output terminal 60 of the pulse train generator12 again through resistor 70 and diode 68. Pulse train generator 12 willbe oscillating thereby generating the recurring pulse train signals A-1or A-2, respectively. In this mode, the output signal appearing atterminal 92 of the time constant circuit 14 will comprise a series ofsequentially occurring increasing and decreasing exponential voltagewaves having their time constant determined by capacitors 52, 54, and76, 78, and resistors 44 and 90, respectively and of differentfrequency, these being shown as wave forms B-1 and B-2 in FIG. 3.

When switch 72 has its armature 74 in the 6b position and switch 48 hasits armature 50 in the 6a position, pulse train generator 12 will againgenerate a series of recurring pulse signals these being shown as waveform A-3 in FIG. 3. However, these signals are now applied throughterminal 6b of switch 72 to the voltage divider resistors 80, 82.Correspondingly, there will be produced a voltage wave form B-3 which isitself a square wave.

The respective ones of the wave forms being generated at output terminal92 will be passed through and amplified by isolation transistor 100 andapplied to the input terminal 164 of the VFO 16.

As stated above, the VFO 16 is a variable frequency, astablemultivibrator circuit. The operation of this type of circuit is wellknown to those skilled in the art and it is sufficient to state that itwill generate a square wave signal having a frequency which is directlyproportional to the voltage applied to its input terminal 164. The valueof the components of the VFO 16 are selected such that its signalfrequency will be within the audio range for all values of voltageapplied to its input terminal 164 by the time constant circuit 14.

VFO 16 also includes a balance control 124. This balance control 124enables adjusting the symmetry of the square wave signal generated bythe VFO circuit 16 whereby the square wave signal will be renderedperfectly symmetrical. Since the power efficiency of the power amplifier18 will be diminished if the square wave signal generated thereby is notsymmetrical, it will now be apparent that the balance control 124provides a simple yet effective means for optimizing the powerefficiency of the power amplifier 18.

Similarly, the addition of the capacitor 160 connected across loadresistor 126 will reduce the switching time of the transistors 130, 134.This in turn reduces the amount of "rounding off" of the square waveproduced by the VFO 16 and again contributes to the efficiency thereof.The output signal from the VFO circuit 16 will therefore be a variablefrequency, constant power square wave signal having a frequency directlyproportional to the voltage applied to the input terminal 164 thereof.This output signal will be either a constant frequency signal wheneverswitches 48 and 72 have their armatures in the 1a, 2a, and 1b, 2bpositions, respectively. This signal will be a manually controlledvariable frequency signal when switches 48 and 72 have their armatures50 and 74 in the 3a, 3b positions or switch 86 is manually operated.When switches 48 and 72 have their armatures 50 and 74 in the 4a, 5a,and 6a, and 4b, 5b, and 6b positions, respectively, the wave formappearing in the output terminal 92 will be the wave form B-1, B-2, orB-3 thereby causing the VFO circuit 16 to produce a variable frequencysignal having frequency envelopes C-1, C-2, or C-3, (FIG. 3)respectively.

The amplifier 166 will amplify the output signal from the VFO 16 andapply this signal to the input winding 216 of coupling transformer 208whenever switches 220 and 222 have their armatures 224, 226 in the 3c,4c, 5c and 6c, and 3d, 4d, 5d, and 6d positions, respectively. Thevoltage of the signal generated by the VFO circuit 16 will again be ofconstant power, and only the frequency thereof varies.

The magnitude of the signal from the VFO circuit 16 and isolationamplifier 166 is sufficient .[.magnitude.]. to drive or bias poweramplifier transistors 180 and 182 individually and alternately betweenfully saturated and fully "off" states. Power transistors .[.180, 182.]..Iadd.268, 270 .Iaddend.because they are diffused based transistors,will switch between their saturated and fully "off" condition with aminimum of "rounding off" of the wave fronts and thereby again adding tothe efficiency of the circuit.

It will be observed that the biasing of transistors 268, 270 is effectedby transistors 180, 182 by reason of their having their bases coupleddirectly to the output circuit of a respective one of transistors 180,182. Consequently, transistors 268, 270 are biased to either aconductive or a non-conductive state directly by the respective one oftransistors 180, 182. That is, transistors 268, 270 will be eitherconductive, when transistors 180, 182 are conductive, respectively, orwill be non-conductive when transistors 180, 182 are nonconductive,respectively. Thus transistors 268, 270 are seen to be automaticallybiased or controlled by the transistors 180, 182.

When the siren 10 is operating in a siren mode, the signals applied tothe bases 202, 204 of power transistors 180, 182 are of sufficientmagnitude to drive the transistors 180, 182 between fully "off" and asaturated, fully "on" condition. Transistors 180, 182 will whensaturated, drive transistors 268, 270 into saturation. When transistors268, 270 are in a saturated, "on" condition they will experience aminimum of heat build up thereby enabling transistors 268 and 270 to beoperated at substantially higher power levels than would be possible ifthey were operated in less than a saturated condition when conductive.Further, when transistors 180, 182, 268, and 270 are being operated in asaturated state, it has been found that transistors 268, 270 willproduce a significant amount of power gain despite the fact that theyare connected in an emitter follower configuration.

When the power amplifier circuit 18 is being used to amplify a signalfrom a radio receiver means (not shown), or from microphone 234, it willbe apparent that transistors 180, 182 and transistors 268, 270 mustoperate as relatively low distortion amplifier devices. Accordingly,when it is desired to use a power amplifier 18 as an audio amplifier forthe radio or microphone signal, it is not possible to drive thetransistors 180, 182, 268 and 270 between fully "off" and fully "on,"saturated states. Rather, transistors 180, 182 still operate toautomatically bias the second pair of transistors 270, 272, but to adegree of conductivity less than or below their saturation levels.Specifically, an audio signal applied to the transistors 180, 182 fromeither the radio input terminals 232 or from the microphone 234 ismaintained at a level substantially below that required to drive thefirst pair of transistors 180, 182 into saturation. Because transistors180, 182 are biased to their threshold of conductivity as describedabove, and by reason of their connection in a push-pull configuration,they will be alternately and individually rendered conductive byalternately phased portions of the input signals from the radio inputterminals 232 or microphone 234. Transistors 180, 182 will in turnrender transistors 268, 270 alternately and individually conductive,respectively, but not saturated. Operating in this manner, it will beobserved that each of the transistors 268, 270 will be operated for a 50percent or less duty cycle and, simultaneously, will operate withrelatively low distortion. Correspondingly, these latter two transistorscan be operated as audio amplifier components and at substantiallyhigher power levels than would be possible were transistors 268, 270statically biased in a conventional manner.

It should further be noted that diffused based transistors are primarilyadapted for use as high power switching transistors which, as stated,are capable of switching between their fully saturated and fully "off"conditions with great speed and exhibit low heat build up whenconducting in a saturated state. It is this characteristic of thediffused base transistor which renders it desirable for use in a highpower, square wave amplifier. However, these same characteristicsnormally render a diffused based transistor unsuitable for use as anaudio amplifier component since the load impedances used therewith toenable the highest power output from the transistor would cause thetransistors to overheat and burn out in a very short period of timeshould the transistors be statically biased for operation in a linearregion as is normally done when the device is used in an audioamplifier. However, in the present circuit, the interconnection of thetwo pairs of power transistors 180, 182 and 268, 270, whereby the secondpair of transistors 268, 270 are automatically biased by the first pairof transistors and whereby the second pair of transistors 268, 270 arerendered non-conductive for at least half of the time, enables the useof these transistors as audio amplifier components at high power levelsand without the danger of transistors 268, 270 being over driven andburned out.

It should be noted that when jumpers 244 and 245 are installed in thecircuit, the VFO circuit 16 will continue to operate and thereby producea siren signal at the output terminal 280 of the power amplifier 18 evenwhen switches 220, 222 have their armatures in the 2c and 2d positions,respectively. However, when the microphone switch 240 is closed, andcorrespondingly microphone terminals 242 are closed, the B+ potentialwill be applied directly to the base of isolation transistor 100rendering it non-conductive. This in turn will cause full B+ potentialto be applied to the bases 130, 134 of the VFO circuit 16 rendering itinactive. It can thus be seen that utilization of the jumpers 244, 245will cause automatic turning "off" of the siren whenever the microphoneswitch 240 is depressed thereby permitting simultaneous use of bothdevices.

When an auxiliary siren circuit (not shown) is connected to auxiliarysiren terminal 88, the interconnecting wires will generally possess asignificant amount of reactance. This reactance would, in turn, normallyresult in degradation of the operation of VFO circuit 16 and possiblycause it to "run away." However, isolation amplifier 100 prevents thisreactance from affecting the VFO circuit parameter.

It can thus be seen that the electronic siren circuit of the presentinvention, by providing means for rendering the square wave signalgenerated within the VFO circuit 16 perfectly symmetrical, by reason ofthe minimization of the "rounding off" of the square wave generatedthereby, and the utilization of diffused base transistors in the poweramplifier circuit produces an electronic siren circuit having a maximumpower efficiency.

In a working model of an electronic siren circuit in accordance with thepresent invention, the following component values were used withresistors followed by an (*) having a 5 percenttolerance.RESISTORS______________________________________34 10 K * 1261.8 K *38 8.2 K 128 56039 22 K 122 1.8 K40 32 K 124 Pot.344 7.5 K * 14022 K *70 3.3 K 142 22 K *80 18 K * 146 22 K *82 10 K * 148 22 K *84 2.2K 188 190 47 K 212 47102 4.7 K 230 1 K118 390 * 236 560 248 1.8K______________________________________POTENTIOMETERS 124 500 ohms. 230100 238 350 (linear)______________________________________CAPACITORS52200 mfd, 10 volt 133 0.022 mfd54 100 mfd, 10 volt 160 0.047 mfd56 39mfd, 10 volt 250 0.1 mfd76 150 mfd, 10 volt 290 200 mfd, 15 volt78 4.7mfd, 10 volt 292 200 mfd, 15 volt132 0.022mfd______________________________________TRANSISTORS30, 42, 100, 1122N4402180, 182 TIP41268, 270 CP2357Amplifier 1662N5307______________________________________DIODES69, 144, 150 IN 457293IN 2030B (Zener)______________________________________

While there have been described above the principles of this inventionin connection with specific apparatus, it is to be clearly understoodthat this description is made only by way of example and not as alimitation to the scope of the invention.

What is claimed is:
 1. In an electronic siren the combinationcomprising:a. means for selectively generating periodically occurringcharging pulses at one of a plurality of predetermined repetition rates,b. time constant determining means coupled to said pulse generatingmeans for generating a corresponding plurality of repeating voltage waveforms in response to said charging pulses, c. means coupled to said waveform generating means for generating a square wave signal having afrequency proportional to the instantaneous voltage of said voltage waveform, d. a power output amplifier circuit including1. a first pair of.[.diffused base.]. transistors connected in a push-pull circuitconfiguration,
 2. first circuit means selectively coupling said firstpair of transistors to said square wave generator for driving said firsttransistors individually and alternately into saturation in response tosaid square wave signal,
 3. a second pair of .[.diffused base.].transistors connected in a push-pull emitter follower configuration,.Iadd.said second pair of transistors being diffused base transistors,.Iaddend.
 4. second circuit means coupling said second pair oftransistors to said first pair of transistors for driving said secondpair of transistors individually and alternately into saturation insynchronism with saturation of respective ones of said first pair oftransistors, and e. impedance matching means for coupling said secondpair of transistors to a sound reproducing device.
 2. The combination ofclaim .[.2.]. .Iadd.1 .Iaddend.wherein said first circuit means includesmeans for applying said square wave signal to the bases of said firstpair of transistors, said second circuit means is coupled to the basesof said second pair of transistors, said second circuit means includesan auto transformer having a pair of input terminals and a pair ofoutput terminals, said first pair of transistors each having a similarone of their collector and emitter connected in common, and the other oftheir collector and emitter connected to said input terminals,respectively, said second pair of transistors having their basesconnected individually to different ones of said output terminals. 3.The combination of claim 1 further comprising means for generating afirst audio signal, said first circuit means further including firstswitch means for selectively connecting said power output circuit to oneof said square wave generators and said audio signal means, means forelectrically biasing the transistors of said first pair of transistorsto their threshold of conductivity whereby said first circuit meansrenders said first pair of transistors individually and alternatelyconductive in proportion to said first audio signal. .[.4. Thecombination of claim 3 further comprising means for electrically biasingthe transistors of said first pair of transistors to their threshold ofconductivity, said first circuit means rendering said first pair oftransistors individually and alternately conductive in proportion tosaid first and said second audio signals in response thereto..].
 5. Thecombination of claim 3 wherein said second circuit means includes meansfor rendering said transistors of said second pair of transistorsindividually and alternately conductive in proportion to the degree ofconductivity of said first pair of transistors when said output circuitis selectively connected to said audio signal means.
 6. The combinationof claim 1 wherein said square wave generating means comprises anastable multivibrator circuit which includes a pair of switchingtransistors, said switching transistors each having a similar one oftheir collector and emitter connected in common, their bases coupled tosaid time constant determining means, and a pair of capacitorsconnecting the other of said collector and emitter of each saidswitching transistor to the base of the mutually opposite one of saidswitching transistors, the switching frequency of said multivibratorbeing proportional to the voltage applied to said bases.
 7. Thecombination of claim 6 wherein said multivibrator circuit includes anoutput circuit having a pulse-shaping means for increasing the switchingspeed of said switching transistors.
 8. The combination of claim 7wherein said multivibrator further includes balancing means forrendering said square wave symmetrical.
 9. The combination of claim 8wherein said balancing means includes a resistor and a potentiometerconnected electrically in series with the collector-emitter circuit ofone of said switching transistors.
 10. The combination of claim 9wherein said pulse-shaping means is a capacitor connected electricallyin series with the collector-emitter circuit of the other of saidswitching transistors.
 11. The combination of claim 1 wherein said firstcircuit means includes means for electrically isolating said poweroutput circuit and said multivibrator circuit.
 12. The combination ofclaim 11 wherein said isolating means comprises a Darlington amplifiercircuit which includes an input terminal coupled to said output circuit.13. The combination of claim .[.18.]. .Iadd.11 .Iaddend.furthercomprising second isolating means for electrically isolating said squarewave generating means from said time constant determining means.
 14. Thecombination of claim 13 wherein said second isolating means includes anisolation transistor having a base, and a collector-emitter circuit,said base being connected to said wave form generating means, saidcollector-emitter circuit being coupled to said square wave generatingmeans.
 15. The combination of claim 1 wherein said pulse generatingmeans is a .[.second.]. multivibrator oscillator.
 16. The combination ofclaim 15 wherein said .[.second.]. multivibrator oscillator includes aplurality of frequency determining elements and .[.second.]. switchmeans for selectively connecting a predetermined one of said frequencydetermining elements into said oscillator circuit thereby determiningthe switching frequency of said multivibrator.
 17. The combination ofclaim 16 wherein said selecting means includes a multiple pole switch.18. The combination of claim 16 wherein said frequency determiningelements are capacitors.
 19. The combination of claim 1 furthercomprising means for selectively disabling said pulse generating meansand means coupled to said time constant determining means for manuallygenerating a plurality of charging pulses, said time constant determingmeans being responsive to said manually generated pulses to generate oneof said wave forms.
 20. The combination of claim 14 further comprisingmeans for coupling an auxiliary siren circuit to said time constantdetermining means.
 21. The combination of claim 20 wherein saidauxiliary siren connecting means is connected to said time constantdetermining means, said second isolating means being connected betweensaid auxiliary siren connecting means and said square wave generator.22. For use in an electronic siren capable of producing selectively,audio and siren signals, a power output amplifier circuit includinga. afirst pair of .[.diffused base.]. transistors connected in a push-pullamplifier circuit configuration having input and output circuits, meansfor selectively biasing said transistors to the threshold ofconductivity whereby audio signals coupled to said input circuit renderssaid transistors alternately conductive in proportion to the amplitudethereof, b. a second pair of .[.diffused base.]. transistors connectedin a push-pull emitter-follower amplifier configuration having input andoutput circuits, .Iadd.said second pair of transistors being diffusedbase transistors, .Iaddend.means coupling the first-mentioned outputcircuit to the second-mentioned input circuit for driving said secondpair of transistors alternately into saturation in synchronism withsaturation of respective ones of said first pair of transistors, c.impedance-matching means for coupling said second pair of transistors toa sound producing device, said impedance matching means having input andoutput circuits the input circuit of which being connected across theemitters of said second transistors, the last-mentioned input circuithaving an impedance appearing across said emitters.
 23. The circuitry ofclaim 22 wherein said coupling means includes an auto transformer havinga pair of input terminals and a pair of output terminals, said firstpair of transistors each having a similar one of the collectors andemitters connected in common and the other connected to said inputterminals, respectively, said second pair of transistors having thebases thereof connected respectively individually to different ones ofsaid output terminals.
 24. The circuitry of claim 23 in which theemitters of said first transistors are connected in common and thecollectors thereof are connected, respectively, to said input terminals,said impedance-matching means including a transformer having primary andsecondary windings, the primary winding being connected between theemitters of the second transistors, a source of voltage connectedbetween the center of said primary winding and the collectors of saidsecond transistors.
 25. The circuitry of claim 24 including asquare-wave generator and a source of audio signals selectively coupledto said input circuit of the first-mentioned amplifier and switch meansfor selecting which of said generator or source is applied to said firstmentioned input circuit.